In vivo models are critical to the understanding of viral diseases. The lack of animal models that display pathology following infection with human immunodeficiency virus type 1 (HIV-1) has hindered our knowledge of the viral mechanisms associated with pathogenesis. The development of severe combined immunodeficient (SCID) mouse/human chimeras has provided novel model systems with which to study the human immune system and the effect of HIV infection on human cells and tissues. These models involve transplantation of various types of human tissues into mice who possess the SCID genetic defect. This defect prevents rejection of the transplanted tissue by the host, thereby providing an in vivo environment for culturing human tissue. Nude mice, which lack a functioning thymus, and hence have not T-cell-mediated immunity, can similarly received certain types of xenogeneic transplants. The CFAR supported Mouse/Human Chimera Core is designed to assist AIDS investigators at UCLA with their research by providing state-of-the-art mouse/human chimera technologies and appropriate animal housing facilities. This facility will supply and house SIC and/or nude mice for AIDS-related research under both Biosafety Level 2 and Biosafety Level 3 conditions. This facility will also provide consultation on the use of chimeric models, as well as construct various mouse/human chimeric animals for distribution to Core users.